System for detecting a change in light intensity

ABSTRACT

Apparatus for inactivating a direct lithographic press in response to the detection of a sheet of paper being undesirably stuck to the lithographic plate in the machine. The paper detection system includes a light source for illuminating the lithographic plate and a photodetector for detecting illumination reflected from the plate. The output of the photodetector is directed to two RC integrating circuits, one circuit having a relatively long RC time constant and the second circuit having a relatively short RC time constant. The integrating circuit with the long time constant follows slow changes in reflectance in the machine, and provides a self-adjusting reflectance reference. The slight contrast caused by the presence of the sheet of paper on the lithographic plate causes the output of the short time constant RC circuit to rise with respect to the output of the long time constant RC circuit. A comparator means coupled to the outputs of both RC circuits detects this increase, and produces an output signal which is directed to an AND gate. The AND gate has a second input for indicating that sheets of paper are being fed through the machine. When the AND gate receives both input signals, it produces an output signal which is directed to a time delayed latch circuit. When the AND gate produces an output signal for a period of time longer than the time delay of the latch circuit, the latch circuit produces an output signal to disable the machine.

United States Patent [191 Wirtz et a1.

1 1 SYSTEM FOR DETECTING A CHANGE IN LIGHT INTENSITY John S. Wirtz, Henrietta; David C. Nellis, Victor, both of NY.

Assignee: Itek Corporation, Lexington, Mass.

Filed: Dec. 26, 1972 Appl. No.: 318,129

Inventors:

US. Cl 250/561, 250/206, 131/130 Int. Cl. G011! 21/32 Field of Search 250/206, 214 R, 219 DQ,

[56] References Cited UNITED STATES PATENTS 7/1972 Mustert 250/219 DQ 4/1969 Kammuller et al. 250/214 R X 11/1967 Blackmer 250/207 X 5/1960 Gilman 250/214 R X 7/1971 Domalski 250/206 Primary Examiner-Walter Stolwein Attorney, Agent, or Firm-Homer 0. Blair; Robert L. Nathans; William C. Roch [57] ABSTRACT Apparatus for inactivating a direct lithographic press May 21, 1974 in response to the detection of a sheet of paper being undesirably stuck to the lithographic plate in the ma chine, The paper detection system includes a light source for illuminating the lithographic plate and a photodetector for detecting illumination reflected from the plate. The output of the photodetector is directed to two RC integrating circuits, one circuit having a relatively long RC time constant and the second circuit having a relatively short RC time constant. The integrating circuit with the long time constant follows slow changes in reflectance in the machine, and provides a self-adjusting reflectance reference. The slight contrast caused by the presence of the sheet of paper on the lithographic plate causes the output of the short time constant RC circuit to rise with respect to the output of the long time constant RC circuit. A comparator means coupled to the outputs of both RC circuits detects this increase, and produces an output signal which is directed to an AND gate. The AND gate has a second input for indicating that sheets of paper are being fed through the machine. When the AND gate receives both input signals, it produces an output signal which is directed to a time delayed latch circuit. When the AND gate produces an output signal for a period of time longer than the time delay of the latch circuit, the latch circuit produces an output signal to disable the machine.

12 Claims, 2 Drawing Figures SYSTEM FOR DETECTING A CHANGE IN LIGHT INTENSITY BACKGROUND OF THE INVENTION The present invention relates generally to reproducing equipment and more particularly pertains to'a direct lithographic press machine wherein a system is included for detecting the undesired presence of a sheet of paper on the lithographic plate. In a direct lithographic press it is possible that a sheet of paper might stick to the plate and be carried into the ink and water meters. If this happens a good deal of damage may result. Conventionally this is prevented by detecting the presence of the paper sheet by its reflectance which is greater than the reflectance of the plate. However, when electrophotographic plate material is used, difficulties arise since the plate is characteristically pink, and there is very little contrast between the plate and the paper sheet. It is thus desirable to have a system which is capable of detecting the small contrast between the reflectance of the white paper sheet and the reflectance of the colored plate.

SUMMARY OF THE INVENTION In accordance with a preferred embodiment, a system is disclosed for detecting a predetermined change in intensity of measured light. Further, the preferred embodiment provides such a system wherein a photodetector means detects incident light. The output of the photodetector is fed to long term averaging means and short term averaging means. The long term averaging means follows slow changes in intensity and provides a self-adjusting intensity reference. A comparator means compares the outputs of the long term averaging means and the short term averaging means and produces an output signal when the output of the short term averaging means rises with respect to the long term averaging means. Further, the preferred embodiment provides such a system wherein the photodetector detects the intensity of light reflected from an area. Also, the preferred embodiment provides such a system for utilization in a reproducing machine to detect the contrast caused by the presence of a paper sheet, and wherein the operation of the machine is altered in response to the detection of the paper sheet. Also the preferred embodiment provides .such a system wherein the long and short term averaging means both utilize a peak averaging technique. Further, the preferred embodiment provides such a system for use in a direct lithographic press, and wherein the lithographic press is shut down in response to the detection of a paper sheet on the lithographic plate. Also the preferred embodiment provides such a system wherein the long term averaging means and the short term averaging means are RC integrator circuits having respectively a long RC time constant and a short RC time constant, and wherein the output of the comparator means is directed to an AND gate having as a further input a signal indicating that sheets of paper are being fed through the machine. Also, the preferred embodiment provides such a system wherein a time delayed latch circuit is coupled to the output of the AND gate to shut down the machine only after the AND gate has produced an output signal for a predetermined minimum period of time.

Although the preferred embodiment is illustrated as detecting intensity changes in reflected light, it should be realized that the teachings of this invention are applicable to detecting changes in the intensity of transmitted light or just ambient light.

One advantage of the preferred embodiment is that the long term averaging circuit provides a self-adjusting reference for the comparator circuit. The long term averaging circuit follows slow changes in the reproducing machine such as changes in the reflectance of the lithographic plate, changes in the illumination characteristics of the light source, drift effects in the detector, or power supply voltage fluctuations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a block diagram of a preferred embodiment of this invention.

FIG. 2 illustrates schematically some of the components shown in block form in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 illustrates a block diagram of a preferred embodiment of this invention. In the preferred embodiment, the system is utilized in. a direct lithographic press. However, it should be realized that the invention has application in other types of reproducing machines and in general where a slight contrast is desired to be detected. Light from a source 10 illuminates the roller 12 in the direct lithographic press which, during operation of the press, carries a lithographic plate 14. At the location where the roller 12 and plate 14 are illuminated, the sheet of paper just printed should have been discarded from direct contact with the plate during normal operation of the press. However, during a malfunction of the machine a paper sheet adheres to the plate at the location illuminated by the light source 10. If the roller 12 continues to rotate, the paper sheet may be carried into the ink and water meters, thereby possibly resulting in a good deal of damage. Therefore, in the case of a malfunction where a sheet of paper is adhering to the plate 14, it would be desirable to detect the presence of the paper and shut down the machine. When an electrophotographic plate is utilized, it is very difficult to detect the paper since the plate is characteristically pink and there is very little contrast between the pink plate and white paper.

The preferred embodiment of the invention uses a peak averaging technique which is able to respond to the small contrast between the white paper and the pink plate. Peak averaging is utilized as it allows the circuit to ignore reflections from nonrelevant darker areas of the roller, such as holes in the roller or grippers mounted on the roller. In alternative embodiments other averaging techniques may be utilized. In the preferred embodiment, the output of photodetector 16 is applied to buffer amplifier 18 which functions to isolate the detector 16 from the rest of the circuit. The output of buffer amplifier 18 is applied directly to a long term peak averaging circuit 20 and to a short term peak averaging circuit 22 through an offset circuit 21. In the preferred embodiment it is desired to detect a rise in the short term averaged output with respect to the long term averaged output. The long term averaged output follows slow changes in the machine such as changes in the detector 16 from the rest of the circuit. The output of buffer amplifier 18 is applied directly to a long term peak averaging circuit 20 and to a short term peak-averaging circuit 22 through an offset circuit 21. In the preferred embodiment it is desired to detect a rise in the short term averaged output with respect to the long term averaged output. The long term averaged output follows slow changes in the machine such as changes in the reflectance of the lithographic plate or changes in the illumination characteristics of the light source or drift effects in the detector. The long term averaging circuit therefore provides a self-adjusting reflectance reference for the comparator circuit 24. Offset circuit 21 reduces the voltage applied to short term averaging circuit 22. Differential comparator circuit 24 compares the outputs of the long and short term averaging circuits 20 and 22. Because of the offset in voltage introduced by circuit 21, differential comparator 24 detects a predetermined rise in the output voltage of the short term averaging circuit with respect to the voltage of the long term averaging circuit which occurs when the voltage difference between the outputs of circuits'20 and 22 is reduced to zero. When this voltage difference is reduced to zero the short term averaged signal has risen a predetermined amount with respect to the long term averaged signal because of the offset in voltage introduced by circuit 21. At that point the output of differential comparator 24 changes to a positive state which indicates that aslig'ht contrast exists on the surface of roller 12.

The positive state of comparator 24 supplies one enabling signal to AND gate 26. AND gate 26 has a second input, indicatedat 29, which also carries a positive voltage when sheets of paper are being fed through the reproducing machine. The system for indicating sheets of paper are-being fed through the machine does not form a part of this invention, and accordingly that apparatus is not illustrated in detail. The possibility that a sheet of paper might undesirably stick to the lithographic plate exists only when sheets of paper are being fed through the lithographic press. Accordingly, AND gate 26 and the signal on line 29 function to ensure that a jam'condition is not indicated at improper times. In addition it provides time for the long term averager to store a propervalue. The output of AND gate 26 is directed to a time delayed latch circuit 28. Time delayed latch circuit, like AND gate 26, is a refinement on the basic invention and produces an output jam signal only after AND gate 26 has produced a signal for a predetermined minimum period of time determined by the time delay pf latch circuit 28. Time delayed latch circuit 28 ensures that transient responses do not jam the lithographic press. In the preferred embodiment, the output of latch circuit 28 is utilized to shut down the press so I that the sheet of paper sticking to the lithographic plate does not get carried into the ink and water meters of the press, thereby possibly resulting in a good deal of damage.

Referring to FIG. 2, there is illustrated a partially schematic diagram of the circuit shown in FIG. 1. The output of photodetector 16 is directed to a buffer amplifier 18 which includes an NPN transistor 30 in an emitter follower circuit. The emitter of transistor 30 is coupled through a diode 34 to long term peak averaging circuit 20. The voltage at the emitter of transistor 30 establishes a voltage drop across resistor 32 which forms the offset circuit 21. Tap 33 takes part of the voltage across resistor 32, and is coupled through a diode 36 to short term peak averaging circuit 22. Long term peak averaging circuit 20 is an integrating circuit consisting ofa resistor 38 through which a capacitor 40 is charged. A second resistor 42 is shunted across capacitor 40 and functions as a bleed resistor. Likewise, short term peak averaging circuit 22 is an integrating circuit and has a capacitor 46 which is charged through a resistor 44. Capacitor 46 is shunted by resistor 48 which functions to bleed capacitor 46. In accordance with the peak averaging technique utilized in the preferred embodiment when the voltage at the emitter of transistor 30 exceeds the voltage across capacitor 40, current flows through diode 34 to further charge capacitor 40. Likewise when the voltage at tap 33 exceeds the voltage across capacitor 46, current flows through diode 36 to further charge capacitor 46. The RC time constants of long term averaging circuit 20 and short term averaging circuit 22 are determined by the values of each of the resistors and capacitors utilized in each circuit. The voltage at the output of short term peak averaging circuit 22 is normally less than the voltage produced by long term averaging circuit 20 because of the offset voltage introduced by resistor 32. When a contrast exists on the surface of roller 12 and moves past photodetector 16, it causes the voltage at the output of short term averaging circuit 22 to rise with respect to the voltage at the output of long term averaging circuit 20. When the voltage from short term circuit 22 reaches the voltage of long term circuit 20, differential comparator circuit 24 changes state and directs a positive output signal to AND gate 26. The remainder of the circuit of FIG. 2 functions in the same manner as explained with respect to the circuit of FIG. 1.

in one embodiment of this invention which was actually built, the press in which the preferred embodiment was implemented had roller 12 rotating at rpm. The. time constant of the long term averaging circuit 20 was selected to be 0.2 seconds, and the time constant of the short term averaging circuit 22 was selected to be 10 seconds. Time delayed latch 28 was designed to produce an output signal after a delay time of approximately 10 seconds.

During operation, jam detection was inhibited for four seconds after the circuit was activated to allow the long term averaging circuit 20 to reach equilibrium. During this four second time period no paper was fed through the machine, and accordingly an activating input was not present on line 29.

While several embodiments have been described, the teachings of this invention will suggest many other embodiments to those skilled in the art.

l claim:

1. A system for detecting a predetermined change in intensity of measured light and comprising:

a. photodetector means for detecting incident light and for producing one output signal indicative of the intensity of said light;

b. long term averaging means coupled to the output of said photodetector means and responsive to said one output signal for producing a signal indicative of a relatively long term averaged output of said one output signal of said photodetector means;

c. short term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively short term averaged output, as compared to said relatively long term averaged output, of said one output signal of said photodetector means; and

d. comparator means, coupled to the outputs of said long term averaging means and said short term averaging means, for detecting a predetermined change in the output of said short term averaging means with respect to the output of said long term averaging means and for producing an output signal upon the occurrence of said predetermined change, said output signal of said comparator means being indicative of said predetermined change in intensity of measured light.

2. A system as set forth in claim 1 and wherein said photodetector means detects the intensity of light reflected from an area.

3. A system for utilization in a reproducing machine to detect the slight contrast caused by the undesired presence of a sheet of paper over a background area and operating by detecting a predetermined change in intensity of light reflected from the background area and comprising:

a. photodetector means for detecting incident light and for producing an output signal indicative of the intensity of said light;

b. long term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively long term averaged output of said photodetector means;

0. short term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively short term averaged output, as compared to said relatively long term averaged output, of said photodetector means;

d. comparator means, coupled to the outputs of said long term averaging means and said short term averaging means, for detecting a predetermined change in the output of said short term averaging means with respect to the output of said long term averaging means and for producing an output signal upon the occurrence of said predetermined change, said output signal of said comparator means being indicative of said predetermined change in intensity of measured light; and

e. means, responsive to the output signal of said comparator means, for altering the operation of said reproducing machine.

4. A system as set forth in claim 3 and wherein:

a. said long term averaging means includes means for producing a signal indicative of the long term averaged peak output of said photodetector means; and

b. said short term averaging means includes means for producing a signal indicative of the short term averaged peak output of said photodetector means.

5. A system as set forth in claim 4 wherein:

a. said reproducing machine is a direct lithographic press reproducing machine; and

b. the system is utilized to shut down the machine.

6. A system as set forth in claim 5 wherein:

a. said long term averaging means includes an RC integrating circuit having a relatively long RC time constant; and

b. said short term averaging means includes an RC integrating circuit having a relatively short RC time constant as compared to the relatively long RC time constant of said long term integrating RC circuit.

7. A system as set forth in claim 6 and including:

a. an AND gate coupled to the output of said comparator means; and

b. means for directing a second input signal to said AND gate for indicating that sheets of paper are being fed through the reproducing machine.

8. A system as set forth in claim 7 and including a time delayed latch means coupled to the output of said AND gate for producing an output signal when said AND gate produces an output signal for a period of time longer than the time delay of said latch circuit.

9. A system as set forth in claim 3 wherein:

a. said reproducing machine is a direct lithographic press reproducing machine; and

b. the system is utilized to shut down the machine.

10. A system as set forth in claim 3 wherein:

a. said long term averaging means includes an RC integrating circuit having a relatively long RC time constant; and

b. said short term averaging means includes an RC integrating circuit having a relatively short RC time constant as compared to the relatively long RC time constant of said long term integrating RC circuit.

11. A system as set forth in claim 3 and including:

a. an AND gate coupled to the output of said comparator means; and

b. means for directing a second input signal to said AND gate for indicating that sheets of paper are being fed through the reproducing machine.

12. A system as set forth in claim 11 and including a time delayed latch means coupled to the output of said AND gate for producing an output signal when said AND gate produces an output signal for a period of time longer than the time delay of said latch circuit. 

1. A system for detecting a predetermined change in intensity of measured light and comprising: a. photodetector means for detecting incident light and for producing one output signal indicative of the intensity of said light; b. long term averaging means coupled to the output of said photodetector means and responsive to said one output signal for producing a signal indicative of a relatively long term averaged output of said one output signal of said photodetector means; c. short term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively short term averaged output, as compared to said relatively long term averaged output, of said one output signal of said photodetector means; and d. comparator means, coupled to the outputs of said long term averaging means and said short term averaging means, for detecting a predetermined change in the output of said short term averaging means with Respect to the output of said long term averaging means and for producing an output signal upon the occurrence of said predetermined change, said output signal of said comparator means being indicative of said predetermined change in intensity of measured light.
 2. A system as set forth in claim 1 and wherein said photodetector means detects the intensity of light reflected from an area.
 3. A system for utilization in a reproducing machine to detect the slight contrast caused by the undesired presence of a sheet of paper over a background area and operating by detecting a predetermined change in intensity of light reflected from the background area and comprising: a. photodetector means for detecting incident light and for producing an output signal indicative of the intensity of said light; b. long term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively long term averaged output of said photodetector means; c. short term averaging means coupled to the output of said photodetector means for producing a signal indicative of a relatively short term averaged output, as compared to said relatively long term averaged output, of said photodetector means; d. comparator means, coupled to the outputs of said long term averaging means and said short term averaging means, for detecting a predetermined change in the output of said short term averaging means with respect to the output of said long term averaging means and for producing an output signal upon the occurrence of said predetermined change, said output signal of said comparator means being indicative of said predetermined change in intensity of measured light; and e. means, responsive to the output signal of said comparator means, for altering the operation of said reproducing machine.
 4. A system as set forth in claim 3 and wherein: a. said long term averaging means includes means for producing a signal indicative of the long term averaged peak output of said photodetector means; and b. said short term averaging means includes means for producing a signal indicative of the short term averaged peak output of said photodetector means.
 5. A system as set forth in claim 4 wherein: a. said reproducing machine is a direct lithographic press reproducing machine; and b. the system is utilized to shut down the machine.
 6. A system as set forth in claim 5 wherein: a. said long term averaging means includes an RC integrating circuit having a relatively long RC time constant; and b. said short term averaging means includes an RC integrating circuit having a relatively short RC time constant as compared to the relatively long RC time constant of said long term integrating RC circuit.
 7. A system as set forth in claim 6 and including: a. an AND gate coupled to the output of said comparator means; and b. means for directing a second input signal to said AND gate for indicating that sheets of paper are being fed through the reproducing machine.
 8. A system as set forth in claim 7 and including a time delayed latch means coupled to the output of said AND gate for producing an output signal when said AND gate produces an output signal for a period of time longer than the time delay of said latch circuit.
 9. A system as set forth in claim 3 wherein: a. said reproducing machine is a direct lithographic press reproducing machine; and b. the system is utilized to shut down the machine.
 10. A system as set forth in claim 3 wherein: a. said long term averaging means includes an RC integrating circuit having a relatively long RC time constant; and b. said short term averaging means includes an RC integrating circuit having a relatively short RC time constant as compared to the relatively long RC time constant of said long term integrating RC circuit.
 11. A system as set forth in claim 3 and including: a. an AND gate coupled to tHe output of said comparator means; and b. means for directing a second input signal to said AND gate for indicating that sheets of paper are being fed through the reproducing machine.
 12. A system as set forth in claim 11 and including a time delayed latch means coupled to the output of said AND gate for producing an output signal when said AND gate produces an output signal for a period of time longer than the time delay of said latch circuit. 